The present invention is a redistribution process for enriching a low-boiling methylsilane mixture in a methylchlorosilane selected from a group consisting of dimethylhydrochlorosilane and trimethylchlorosilane. The process comprises contacting a low-boiling methylsilane mixture, resulting from the contact of methyl chloride with silicon, with alumina under non-equilibrium conditions at a temperature greater than about 150.degree. C. The present inventors have discovered that the concentrations of dimethylhydrochlorosilane and trimethylchlorosilane reach maximum levels under non-equilibrium conditions occurring at temperatures greater than about 150.degree. C. The present process is also useful for removing olefin and chlorocarbon organic contaminants from the low-boiling methylsilane mixture.
Methylchlorosilanes are the basic monomers from which a wide variety of organosilicon containing fluids, rubbers, and resins are formed. Commercially these methylchlorosilane monomers are produced by a process typically referred to as the "direct process." In the direct process, methyl chloride is reacted with silicon in the presence of a catalyst comprising copper. The process was first described by Rochow, U.S. Pat. No. 2,380,995. Commercially, production of polydimethylsiloxanes represent the highest volume use of methylchlorosilanes from the direct process. Therefore, considerable effort has been directed toward optimizing the direct process to produce dimethyldichlorosilane. Such optimization efforts are described in, for example, Ward et al., U.S. Pat. No. 4,500,724.
Despite all attempts to optimize the direct process for dimethyldichlorosilanes, the effluent from the reactor is still a mixture of methylsilanes and higher boiling materials which can include disilanes, polysiloxanes, silylmethylenes, and particulates. Typically the effluent exiting the reactor is distilled to separate the methylsilanes from the higher-boiling materials. The methylsilane distillate comprises a mixture having dimethyldichlorosilane as a major component and minor components comprising, for example, tetramethylsilane, dimethylhydrochlorosilane, methylhydrodichlorosilane, trimethylchlorosilane, and methyltrichlorosilane. These minor components can represent as much as 15 weight percent of the monosilanes produced in the direct process. Commercial demand for methylsilanes can at times make it desirable to increase the proportion of certain of these minor component methylsilanes, such as dimethylhydrochlorosilane and trimethylchlorosilane, in the direct process effluent. Although it is possible to alter the ratios of methylsilanes exiting the direct process reactor by changes to the process, manufacturers are reluctant to risk upsetting a process on which considerable resource has been expended to optimize for production of dimethyldichlorosilane. Therefore, methods are desirable to alter the product mix from the direct process exterior to the direct process reactor. The present method provides a process whereby the concentration of dimethylhydrochlorosilane and trimethylchlorosilane in the effluent from the direct process can be increased. The method comprises contacting a low-boiling methylsilane mixture, resulting from the reaction of methyl chloride with silicon, with alumina under non-equilibrium conditions at a temperature greater than about 150.degree. C. Unexpectedly, the present inventors have found that under non-equilibrium conditions higher concentrations of dimethylhydrochlorosilane and trimethylchlorosilane can be obtained in the low-boiling methylsilane mixture than would be predicted by standard equilibrium calculations.
The present process is also useful for removing olefin and chlorocarbon organic contaminants from the low-boiling mixture. Under process conditions, olefin and chlorocarbon organic contaminants can be reacted with silicon-bonded hydrogen to convert the contaminants to saturated alkanes. Therefore, the present invention provides a process where redistribution of methylsilane and removal of olefin and chlorocarbon organic contaminants can be effected in a single step.
Wynn, U.S. Pat. No. 3,704,260, teaches that trimethylchlorosilane and methylhydrodichlorosilane can be rearranged in the presence of aluminum trichloride to form dimethylhydrochlorosilane. The rearrangement is preferably carried out at a temperature of about 110.degree. C. to 140.degree. C. for about 0.25 to 8 hours.
Viego et al., U.S. Pat. No. 3,769,310, teach the redistribution of methylhydrodichlorosilane with trimethylchlorosilane to produce dimethylhydrochlorosilane using a catalyst selected from a group consisting of AlCl.sub.3, KAlCl.sub.4, and BF.sub.3. Viego et al. teach the process is to be run until equilibrium conditions are established at a temperature within a range of 50.degree. C. to 250.degree. C. and a time within a range of 2 to 7 hours.
Marko et al., U.S. Pat. No. 4,774,347, teach a process for reducing the chlorocarbon content of alkylsilanes. The process comprises contacting crude alkylsilanes containing as a minor portion chlorocarbons, and a hydrogen-containing silane with a catalyst that facilitates the reaction of the chlorocarbons with the hydrogen-containing silane to convert the chlorocarbons to an alkane. Marko et al. teach that alumina may be a useful catalyst in the process and that during conduct of the process some rearrangement of more highly alkylated silanes with other alkylhalosilanes may occur. Marko teaches the process can be run at a temperature within a range of about 25.degree. C. to less than 150.degree. C.